The tectono-metamorphic event at 2 Ga in the Limpopo Belt and the resetting behaviour of chronometers at high temperature

Abstract

Structural observations with garnet and monazite dates from the Messina area, Central Zone of the Limpopo Belt, are presented, as well as Rb-Sr and zircon ages from the Transition Zone between the Triangle shear zone and the Northern Marginal Zone. It is shown that the 2.57 Ga Bulai pluton NW of Messina is deformed by a later high grade event involving minor local remelting. Within samples, monazite concordia ages broadly concur with garnet-feldspar Pb-Pb results. In some samples they document full resetting of both chronometers at 2.0 Ga, but in some they display mixed ages indicating pre-existing parageneses of at least 2.6 Ga. Unlike the 2.0 Ga metamorphism, the age of earlier events could not be well constrained. Reset ages can be related to the presence of melt, rather than deformation, at 2.0 Ga, but no strict rule can be applied. In the Transition Zone, Rb-Sr whole rock and zircon dating did not reveal the 2.0 Ga event. A suite of granites, gneisses and migmatites yielded an apparent magmatic Rb-Sr age of 2.59 ± 0.04 Ga, whereas zircon U-Pb dating gave upper intercepts between 2.22 and 2.35 Ga, of uncertain significance. An overview in histogram form of available age results from the Limpopo Belt shows peaks at 2.6 - 2.7 Ga in the Northern and Southern Marginal Zones but not in the Central Zone, and a peak at 2.0 in all three zones, indicating that the 2.0 Ga event is the first episode common to the three zones of the Limpopo Belt.